DEL DIOS – San Diego is not known as a hydroelectric hotbed. The image of large dams with gigawatt-churning turbines is more typical in the Pacific Northwest.

But one component of the tunnel under construction between Lake Hodges and the Olivenhain Reservoir is a 40-megawatt power plant. One megawatt powers about 1,000 typical

Here's how it works:

The tunnel's purpose is to shift water between the reservoirs.

During dry spells, the San Diego Water Authority will be able to refill Lake Hodges from the Olivenhain Reservoir, which receives water from the California aqueduct. During wet winters when Lake Hodges fills, the authority will pump water uphill through the tunnel into the Olivenhain Reservoir.

The process will take energy because the water will have to be pumped up a 700-foot incline through a 6,300-foot pipe.

Moving water in the opposite direction will require only gravity, and that's where the hydroelectric potential comes into play.

When the water flows down the pipe, it will turn a pump turbine, and voilà, hydroelectric power.

“When the water flows back down, we'll generate 40 megawatts of electricity,” said Joe Bride, the water authority's tunnel project manager. “We have an agreement (with San Diego Gas & Electric) to use it for reserve power when the grid needs power.”

The money the authority makes from generating power will help pay for the $104 million project.

A small hydroelectric power plant also operates in Escondido, generating electricity as water is moved from Lake Wohlford through the plant to Dixon Lake.